Helium changes to a liquid when it is
cooled to -268.6° C, about 4° C above absolute zero. It
is the only element that cannot be changed to a solid by
cooling alone. It must be both cooled and compressed. It
freezes at -272° C under a pressure of 26 times
atmospheric pressure.

Liquid helium, unlike most other
liquids, conducts heat extremely well, flows toward
relatively warm places, and expands instead of
contracting when it cools. Liquid helium forms a thin
film over everything it touches. This film can act as a
siphon, carrying the helium over the side of a container
to a lower level.

Occurence and Sources

Helium makes up about 23 per cent of
the matter in the visible universe. The Sun and other
stars are made mostly of helium and hydrogen, but helium
makes up only a small fraction of the Earth's matter.

Most of the world's helium comes from
five natural gas fields in the United States -- the
Cliffside field in the Texas Panhandle; the Greenwood
field in Kansas and Colorado; the Hugoton field in
Kansas, Oklahoma, and Texas; the Keyes field in Oklahoma;
and, the Panhandle field in Texas.

Helium is purified by cooling the
natural gas until all gases except helium, argon, hydrogen, and nitrogen change to liquid.
Hydrogen is then burned out of the remaining mixture, and
argon is absorbed by charcoal at low temperatures.
Nitrogen often remains in helium as an impurity. Helium
that is 99.995 per cent pure is called grade A
helium; crude helium contains about half helium
and half nitrogen.

Uses

About three-fourths of the helium
produced in the United States is used by federal
agencies. The government's chief use of helium is in
maintaining the proper pressures in rockets. Pressure
must be maintained in rocket fuel tanks during flight, or
the thin walls of the large tanks might collapse as the
fuel drains from them. Helium also produces the pressure
that forces fuel into rocket pumping systems.

The largest industrial use of helium is
in heliarc welding, a type of electric arc
welding. The inert helium keeps oxygen in the air from
reaching the metal. If oxygen reaches the metal, it may
cause the metal either to burn or to corrode. Helium is
also used to prevent chemicals from reacting with other
elements during storage, handling, and transportation.

The most well-known use for helium is
the inflation of balloons. Because helium is lighter than
air, balloons can rise to high altitudes.

History

Evidence of helium in the Sun was
discovered by English astronomer Sir Norman Lockyer in
1868. Lockyer invented the name helium from the
Greek word helias, meaning sun. Helium
was first found on the Earth in 1895 when the Scottish
chemist Sir William Ramsay and the Swedish chemists Nils
Langler and Per Theodor Cleve found it in the mineral
clevite.